In recent years, electric automobiles and hybrid automobiles driven by motors using electric power supplies such as a fuel cell have been developed. A fuel cell is a cell which obtains electric energy by, for example, supplying fuel gas such as hydrogen and oxidant gas such as oxygen to an electrolyte film. The fuel cell has a high power-generating efficiency, and has attracted attention as an environmentally friendly electric power supply.
In some cases, vehicles with a fuel cell are provided with a battery such as a secondary cell in addition to the fuel cell, in consideration of possible loss of electric power supply from the fuel cell or of reduced response from the fuel cell. For example, an electric power supply system having a battery connected to a motor in parallel with a fuel cell has been disclosed. In such a case, where a battery is disposed in parallel with a fuel cell, a DC-DC converter is generally provided between the battery and the motor for matching the voltages of the fuel cell and the battery or for retrieving regenerated energy.